The RAS/ERK MAP kinase pathway transduces signals from multiple receptors in a wide variety of cell types. Over the past 10 years, germ line gain-of-function mutations in multiple pathway components were found in a set of related autosomal dominant human genetic disorders, the RASopathies. These include Noonan Syndrome (NS), caused by PTPN11, SOS1, KRAS, NRAS, SHOC2 or RAF1 mutations, LEOPARD syndrome (LS), caused by PTPN11 mutations, Costello Syndrome (CS), caused by HRAS mutations, and Cardio-Facial- Cutaneous-Syndrome (CFCS) (caused by BRAF, MEK1 or MEK2 mutations). Although these syndromes share several phenotypes, most notably short stature, facial dysmorphia and various cardiac developmental defects, but they also vary considerably. The central tenets of our research are that: a) phenotypic similarities in RASopathies reflect enhanced RAS/ERK signaling, and can reveal key insights into the specific role of this pathway in the affected cell/tissue type; and b) phenotypic differences are due to tissue-specific differences in the function of pathway components and/or feedback pathways. Our long-range goal is to use mouse models of RASopathies to uncover nuances of Ras/Erk pathway regulation in health and disease, with a particular focus on their cardiac consequences. In the initial funding period, we developed global and/or inducible knock-in models for all of the RASopathies, and in some cases, for multiple alleles of a syndrome. We identified the cell-of- origin and cellular defect underlying valvuloseptal defects in Ptpn11 mutant NS, and found that, by contrast, a kinase-activated Raf1 allele (L613V) causes hypertrophic cardiomyopathy (HCM). Also as in humans, our LS model developed HCM. Strikingly, Erk activation was enhanced in L613V/+ hearts and HCM (and other NS features) was reversed by post-natal MEK-I treatment, whereas Akt/mTorc1 activity was increased in LS hearts, and could be reversed by rapamycin. For this continuation proposal, we have assembled a multi-disciplinary PI team to elucidate the detailed molecular and cellular basis for Raf1 mutant HCM, using genetic, proteomic, physiological and proteomic approaches. Aim 1 uses our inducible Raf1L613V knock- in mice and cell-specific Cre recombinase lines to assess the contributions of cardiomyocytes and cardiac fibroblasts to HCM and the physiological consequences. Aim 2 uses candidate and unbiased proteomic/modeling approaches to identify key Erk targets in Raf1 mutant HCM. Aim 3 comprises further pre-clinical validation studies of MEK-inhibitors and other therapeutic agents for NS-associated HCM.